Low dosages of quinidine have recently been identified as potent inhibitors of the cytochrome P450 responsible for the polymorphic biotransformation of encainide to its active metabolites. This project tests the hypothesis that inhibition of P450 dbl may therefore lead to accumulation of parent drug and alteration of pharmacologic response. Patients with P450 dbl phenotype previously established will be evaluated during chronic therapy with low doses of encainide, low doses of quinidine.